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Rain Relenting; Above-Normal Temps And A Partly Sunny Weekend Ahead

Elaine Thompson
AP Photo
A pedestrian hurries along in the rain Wednesday morning, Sept. 24, 2014, in Seattle.

Fall came in with a vengeance this week. An early atmospheric river dumped huge amounts of rain into the Northwest over the last few days, but it’s tapering off just in time for the weekend, says KPLU weather expert Cliff Mass.

“It actually looks like a decent weekend,” Mass said. 

A Nice Weekend Ahead

Friday began with rain that will likely stay through the early afternoon, said Mass, "but I think that’s going to fade out later in the day."
Mass says Saturday and Sunday should be partly sunny and generally dry with temperatures on Saturday getting up as high as 70 and on Sunday into the lower 70s.

“So really quite decent,” Mass said, adding that this time of the year, the average high at SeaTac is 68. “So this is above-normal temperatures.”

Then a weak front will move in Monday, which will cause showers to develop during the late morning and the afternoon. 

“So things will go downhill then,” Mass said.

But all is not lost. Mass says there may be another ridge developing later in the week, bringing back the sun and warmth.

“So we’re back to a pattern where we’re going to have rain, breaks, rains, breaks,  which is very typical for this time of the year,” Mass said.

What Caused The Big Rains

Just in time for the start of fall, Mass noted on his blogThursday that 1 to 4 inches of rain fell over the Northwest in just a couple of days.

“After what we had last night, it’s more like 2 to 4 inches,” he said on Friday.

The reason for most of it? “An early atmospheric river,” Mass said.

He says when moisture coming out of the tropics and sub-tropics moves into the mid-latitudes, it tends to be concentrated into relatively narrow plumes of moisture.

“And we call these atmospheric rivers,” Mass said.

Our version of the atmospheric river is often called the Pineapple Express, because it originates near Hawaii. Mass says we had one of these this week, as a large upper level trough developed over the eastern Pacific and pulled a big plume of moisture into the Northwest, which gave us several days of rain, some of it quite heavy.  

“So, we see these atmospheric rivers during the winter time, but this is a relatively early one and there’s a lot of precipitation, particularly on the southern part of the Olympics and North Cascades,” Mass said. “Some of those folks got over 2 inches. Some got over 3 or 4 inches."


The weekly KPLU feature "Weather with Cliff Mass" airs every Friday at 9 a.m. immediately following BirdNote, and twice on Friday afternoons during All Things Considered. The feature is hosted by KPLU Environment Reporter Bellamy Pailthorp. Cliff Mass is a University of Washington Professor of Atmospheric Sciences, a renowned Seattle weather prognosticator, and a popular weather blogger. You can also subscribe to a podcast of “Weather with Cliff Mass” shows.


Bellamy Pailthorp covers the environment for KNKX with an emphasis on climate justice, human health and food sovereignty. She enjoys reporting about how we will power our future while maintaining healthy cultures and livable cities. Story tips can be sent to